public class LayerDocument : Document 

{
    //Get the LayerDocument from a lyrx file
    var layerDoc = new LayerDocument(@"E:\Data\SDK\Default2DPointSymbols.lyrx");
    //Get the CIMLayerDocument from the LayerDocument and use it to create LayerCreationParams
    var createParams = new LayerCreationParams(layerDoc.GetCIMLayerDocument());
    //Create a FeatureLayer using the LayerCreationParams
    //Note: Needs QueuedTask to run
    LayerFactory.Instance.CreateLayer<FeatureLayer>(createParams, MapView.Active.Map);
}

{
    //Note: Call within QueuedTask.Run()
    var lyrDocFromLyrxFile = new LayerDocument(@"d:\data\cities.lyrx");
    var cimLyrDoc = lyrDocFromLyrxFile.GetCIMLayerDocument();

    //modifying its renderer symbol to red
    var cimSimpleRenderer = ((CIMFeatureLayer)cimLyrDoc.LayerDefinitions[0]).Renderer as CIMSimpleRenderer;
    cimSimpleRenderer?.Symbol.Symbol.SetColor(new CIMRGBColor() { R = 255 });

    //optionally save the updates out as a file
    lyrDocFromLyrxFile.Save(@"c:\data\cities_red.lyrx");

    //get a json representation of the layer document and you want store away...
    var aJSONString = lyrDocFromLyrxFile.AsJson();

    //... and load it back when needed
    lyrDocFromLyrxFile.Load(aJSONString);
    cimLyrDoc = lyrDocFromLyrxFile.GetCIMLayerDocument();

    //create a layer and add it to a map
    var lcp = new LayerCreationParams(cimLyrDoc);
    var lyr = LayerFactory.Instance.CreateLayer<FeatureLayer>(lcp, map);
}

{
    IEnumerable<FeatureLayer> featureLayers = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>()
      .Where(l => l.ShapeType == esriGeometryType.esriGeometryPoint);
    var layerFile = @"C:\Data\SDK\UniqueValuePointLayer.lyrx";
    foreach (var featureLayerToSymbolize in featureLayers)
    {
        //Get the Layer Document from the lyrx file
        var lyrDocFromLyrxFile = new LayerDocument(layerFile);
        var cimLyrDoc = lyrDocFromLyrxFile.GetCIMLayerDocument();

        //Get the renderer from the layer file
        var rendererFromLayerFile = ((CIMFeatureLayer)cimLyrDoc.LayerDefinitions[0]).Renderer as CIMUniqueValueRenderer;

        //Apply the renderer to the feature layer
        //Note: If working with a raster layer, use the SetColorizer method.
        featureLayerToSymbolize?.SetRenderer(rendererFromLayerFile);
    }
}

{
    LayerDocument layerDocument = new LayerDocument(layer);
    //Note: Run within QueuedTask
    layerDocument.Save(@"c:\Data\MyLayerDocument.lyrx");
}